Variable resistance control



Jan. 13, 1959 w F, DEHN ETAL 2,868,931

VARIABLE RESISTANCE CONTROL Filed June l1, 1956 2 Sheets-Sheet 1 I4 /3INVENToR /4 mi, 7. @La

5 46 5l 465,634@ 53 ATTORNEY Jan. 13, 1959 w- F, DEHN ETAL 2,868,931

VARIABLE RESISTANCE CONTROL Filed June 11, 1956 2 Sheets-Sheet 2 ,'7 //VI l faux/M ATTORNEY UnitedStates Patent C) VARIABLE RESISTANCE CONTROLWilliam F. Dehn, Wauwatosa, and Clarence R. Dickinson, Greendale, Wis.,assignors to Allen-Bradley Company, Milwaukee, Wis., a corporation ofWisconsin Application June 11, 1956, Serial No. 590,427

7 claims. (ci. :o1-ss) This invention relates to variable resistanceapparatus that may be employed as a potentiometer or rheostat, `and itmore specifically resides in apparatus havingy a rotatable operatingshaft for moving a brush along an arcuate resistance track, wherein itis one aspect of the invention to provide a bias spring for the brushthat has reaction forces, acting upon the shaft, which are distributedin such manner that the resultant torque, acting to cause a tilt of theshaft, is a minimum. Another aspect of the invention resides in a biasspring that carries the brush and resiliently bears upon it, to urge thebrush not only against the resistance track, but also radially outwardagainst a pair of radially converging engagement faces that. formintegral parts of the spring. In addition, the invention provides forterminal members securely mounted by the casing of the apparatus,whereby such terminal members are isolated mechanically from theresistance track and brush.

When adjustment of a variable resistance is made by grasping and turningthe operating shaft, subsequent release of the shaft may be accompaniedby a slight shift of the shaft that alters the setting that had beenselected. Such a shift is due, primarily, to spring forces within theresistance apparatus that cause a tilt of the shaft within its mounting.Although the shaft may be mounted in a bushing with a close fit, thedegree of tilt permissible may be suiicient to create a shift in brushposition when the shaft is disengaged, that causes a perceptible changein the resistance value selected. This is particularly evident inapparatus employed in circuits having exacting requirements for thecircuit parameters, for the adjustment of the Variable resistance mustin such instances be precise.

In the present inventionthe reaction force of the bias spring, forcreating contact pressure, is transmitted to the operating shaft in amanner that the resultant net torque acting to apply a tilt to the shaftis at or near the vanishing point. To accomplish this minimization ofnet torque the bias spring is placed in stress to react on both thebrush and the shaft, as has been conventional practice in prior devices.The spring, however, extends about the shaft and bears upon thestationary base of the apparatus at more than one point. Between thesepoints of bearing the spring reacts against the shaft and the reactionpoints are positioned so that the resultant net torque of the forcesacting to produce a tilt of the shaft is at, or near, the Vanishingpoint. In this manner if the control shaft be grasped and adjusted byturning to a selected position there Will be no tendency for the shaftto shift, in response to internal torques, as one disengaging from theshaft.

vA further feature of the apparatus shown and described herein lends theapparatus to industrial control applications, where handling is oftenexcessively abusive, as contrasted with laboratory usage and the usualtreatment of communication equipment. The terminals, to whichconnections are made with other circuit components, take a form thatwill withstand abuse and severe treatment,

ICC

and they are mechanically separated from nely constructed interior partsto isolate such interior parts from abusive treatment that may bedelivered to the terminals and the casing for the apparatus. Relativelylarge terminals are selected and these are engaged by the outer casingto prohibit a movement of the terminals independent of the casing. Aflexible conductor connects each terminal with the interior elements. Inthis manner the terminals become a part of the casing isolatedmechanically from the interior, in that forces delivered to theterminals are transmitted to the casing rather than the electricallyconductive elements within the casing.

It is an object of this invention to provide a variable resistance ofrugged construction adapted for industrial control applications.

It is another object of this invention to provide a variable resistancein which adjustments of the control shaft, and hence of the resistance,are retained after disengagement from the control shaft.

It is another object of this invention to provide a variable resistancein which wear between the control shaft and its supporting bushing isminimized.

lt is another object of this invention to provide a variable resistancein which the exposed terminals for connection with other circuitcomponents are isolated mechanically from interior electrical parts toeliminate transmission of abusive forces directed upon the terminals tothe interior.

1t is another object of this invention to provide a variable resistancewith a set of terminals arranged adjacent one another at one side of theapparatus that are each insulated from the others for insuring a highbreakdown voltage rating.

It is another object of this invention to provide a variable resistancein which heat generated in the resistance track is quickly dissipated toenhance the rating of the apparatus.

It is another object of this invention to provide a variable resistancein which forces tending to cause a tilt of the shaft within its bearingsupport are minimized.

It is another object of this invention to provide a variable resistancein which a positive bearing pressure between the brush and theresistance track is insured at all times.

The foregoing and other objects and advantages of this invention willappear from the description to follow. In

the description reference is made to the accompanying drawings, whichform a part hereof, and in which there is shown by way of illustrationand not of limitation a particular form in which the invention may beembodied.

in the drawings:

Fig. l is a side view in elevation and in section of variable resistanceapparatus embodying the invention,

Fig.' 2 is a rear View in elevation of the apparatus shown in Fig. 1taken through the plane 2 2,

Fig. 3 is a rear view in section of the apparatus taken through theplane 3 3 indicated in Fig. l,

Fig. 4 is a view in section of the apparatus taken through the plane 4 4indicated in Fig. 3,

Fig. 5 is a front view of the apparatus taken through the plane 5 5indicated in Fig. 1,

Fig. 6 is a side View in elevation of a portion of the apparatus,

Fig. 7 is a view in section of that portion of the ap. paratus shown inFig. 6 taken through the plane 7-7,

Fig. 8 is a view in perspective of the spring and brush carrier forminga part of the apparatus,

Fig. 9 is a view in perspective of the conductive brush forming a partof the apparatus, and

Fig. 10 is a view in perspective of the bias spring forming a part oftheapparatus. j

Referring now to the drawings, the variable resistance apparatusembodying the invention, as shown, has a molded insulating base 1 formedof a synthetic resin comprising ingredients as already known andunderstood by ythose skilled in the art. An arcuate resistance track 2is exposed about the periphery of the rear face of vthe base 1 andconcentric with the resistance track 2 is a circular collector track 3.The track 3 is also exposed as apart of the rear face of the base 1. Theelectrically conductive tracks 2 and 3 each comprise a shallow layer offinely divided conductor particles dispersed and embedded within themolding material comprising the base 1. An example of a substance thatmay be employed for the particles, and which is also known to thoseskilled in the art, is calcined carbon black. In the resistance track 2the particles are dispersed in such quantity that the desired resistancefor the apparatus is attained, and in the collector track 3 theparticles are more dense to provide a conducting track having negligibleresistance as compared to that of the resistance track 2.

Embedded in the molded insulating base 1 is a heat extractor 4 composedof a thin sheet of heat conductive metal formed into an arcuateconfiguration, as is clearly shown in Fig. 3. The heat extractor d ispositioned with an edge disposed closely to and immediately to the frontof the arcuate resistance track 2, and emerges at the front of the base1 with a set of five ears 5 that are tightly clinched over a iiange 6 ofa bushing 7. The ears 5 form a heat conductive path extending from theembedded portions of the heat extractor d to the bushing flange 6 toconduct heat rapidly away from the interior of the base 1, where theheat generated Within the resistance track 2 might otherwise develop hotspots limiting the rating of the apparatus. The bushing 7 and flange 6are also formed of heat conductive metal, and thereby provide a heatsink capable of absorbing substantial quantities of heat. This heatsink, in turn, is in heat conductive relation to a metallic front plate8 that is adapted to be brought up tightly against a metallic mountingpanel, such as designated by the numeral 9 in Fig. l. The panel 9 is nota part of the apparatus of the invention` but is illustrative of atypical mounting for the invention. To position the apparatus a nut 10threadedly received by the bushing 7 is brought up tight. In thisfashion heat is rapidly drawn away from the apparatus so that thelnterior of the base 1 may be retained at a relatively low temperature.

A front housing wall 11 of sheet insulating material is interposedbetween the front of the base 1 and the afterside of the bushing flange6. The housing wall 11 is retained tightly in position by the clinchingof the heat extractor ears 5 over the front face of the bushing flange6. There is provided a molded cover 12, which is open at the front andbrought un tightly against the front housing wall 11 to form with thewall 11 a casing in which the base 1 resides. The cover 12 has a set ofthree slotted ears 13 disposed about its forward edge` and a finger 14extending from the edge of the front plate 8 1s inserted through eachear 13. Each of the fingers 14 is bent baclcwardly over its associatedear 13 to retcin the front plate 8 in place, and also to maintain thecover 12 tightl,I against the front housing wall 11. The cover 12 willbe described in additional detail hereinafter.

A rotatable control shaft 15 extends from the exterior through thebushing 7 with a close fit to the interior of the apparatus. A springand brush carrier 16 is mounted within the cover 12 upon the shaft 15 tothe immediate rear of the base 1. The carrier 16 is formed of a moldedinsulating material with an intricate configuration as shown in Fig. 8.The forward end of the carrier 16 comprises a sleeve 17 extendingthrough a circular opening in the base 1 and fitting closely about theshaft 15 to provide insulation between the shaft 15 and the conductivecollector track 3. At the after terminaion of the sleeve 17 the brushcarrier 16 extends radially outward r surface 38 with a point to pointcontact.

to form a central boss portion 18 cut away at 19 to receive a conductivebrush 2f?. The boss portion 18 is also recessed to form a pair ofabutments 21 that are on opposite sides of the shaft 15 and which lieaiong a common diameter. A shoulder 22 continues radially outward fromeach abutment 2.1 and a circular fiange 23 extends circumferentiallybetween the shoulders Z2 to form a back for the spring and brush carrier16. As is shown in Fig. l, the carrier 16 is formed with a key 24extending within a complementary keyway 25 in the shaft 15, therebyproviding a coupled rotation of the shaft 15 and the carrier 16.

A stop member 26, of sheet material, shown in Figs. l, 2 and 6, fitsagainst the after face of the fiange 23 of the brush carrier 16 and isprovided with a key 27 fitting within the keyway 25, so as also to berotatable with the control shaft 15. The stop member 2.6 has a turnedend 26 that will strike against a stop abutment ZS formed integrallywithin the cover 12. in this fashion rotation of the shaft 15 and itsassociated spring and brush carrier 16 is limited, for a purpose as willbe hereinafter set forth. A small collar 29* is seated on the after endof the control shaft 15 and has a set screw 30 brought up within thekeyway 25 to retain the carrier 16 and stop member 26 in position.

A bias spring 31, clearly shown in Fig. l0, encircles the boss portion18 of the spring and brush carrier 16. The bias spring 31 has thegeneral form of an open ring and is composed of flat spring materialsuch as beryllium copper. Extending radially inward is a pair offulcruins 32 that fit in the recessed abutments 21 of the carrier 16.Each fulcrum 32 is slightly dished with the convex side resting againstan abutment 21, so that a line of contact is established between theabutments 21 and the fulcrums 32 that coincides with a shaft diameter.fn this fashion pressure exerted by the spring upon the carrier 16 isapplied along a definite well defined line. In quadrature with thefulcrums 32 is a button 33 that is attached to the spring 31 in aposition to ride upon the rear f acc of the base 1, as shown in Figs. land 6. The button 33 is selected of a material having good bearingqualities to insure a smooth ride along the surface of the base 1, whichlikewise is smooth to present -a desirable bearing surface.

The portion of the spring 31 diametrically opposite the button 33 isformed with a pair of radially outer brush engaging fingers 34 and aradially inner brush retaining finger 35. Each of the fingers 34, 35 isturned in a direction substantially normal to the plane of the spring31. in addition, the outer fingers 34 are closely spaced with the majorsurfaces thereof set in planes that converge in a radially outwarddirection.

The brush 2t), as seen in Fig. 7, is generally triangular inconfiguration, and has a pailof side faces 36, one of which is clearlyshown in Fig. 9, which converge toward one another in similar fashion asthe surfaces of the fingers 34. The side faces 36 may then be receivedbetween the fingers 34, with a firm fit that causes the brush Zt) toclosely follow rotational movements of the spring 31. Thus, upon arotation of the shaft 15, and hence of the bias spring 31, the brush Zt)will be rotated without lost motion between the spring 31 and brush 20.The brush retaining finger 3S of the spring 31 extends about the innerend of the brush 2f), which inner end is recessed at 37, to precludeescape of thc brush 2f) in case of an accidental Jiar or similar impactdelivered to the apparatus.

The back of the brush 20, as shownin Fig. 9, is crowned with a convexsurface 33 and has a marked slope, clearly observed in Figs. l and 6.The slope at the back of the brush 2U imparts a radially outwarddivergence Yfrom the rear face of the base 1. A ridge 39 is formed inthe spring 31 to bear against the crowned rIhus, with the spring 31bearing upon the sloping surface 38 the brush 20 will be urged radiallyoutward, upon a rotation of the shaft 15, as well as against theconductive tracks 2, 3. The brush 20 is then firmly held between thebrush engaging fingers34 of the spring 31.

The front face of the ybrush 20 is provided with a resistor contactingfoot 40 that slides upon the arcuate resistance track 2. At the radialinner extent of the front face of the brush 20 is a pair of collectorengaging feet 41 disposed on opposite sides of the finger 35. The feet41 bear upon the circular collector track 3, so that the |brush 20electrically bridges between the tracks 2 and 3. To serve the purpose ofan electrical bridge the brush 20 is composed of a hardened conductivematerial, such as carbon that presents negligible resistance to currentow. lThe resistance track engaging foot 40 may be swung along the entirearcuate length of the resistance track 2, and as an end of the track 2is approached the stop member 26 engages the stop abutment 28. Thetravel of the brush 20 is thus limited to prohibit overrunning the endsof the arcuate track 2.

As viewed in Fig. 6, the bias spring 31 appears as a beam fixed at itsends by engagement with the rear face of the base 1. The upper endportion of the spring 31 bears through the button 33 against the base 1,and the lower end portion of the spring 31 bears through the Ibrush 20against the base 1. The spring 31 is loaded, that is retained in astressed condition, by disposing the abutments 21 ofthe carrier 16 in aposition that warps the spring 31. The reaction of the spring 31 againstthe abutments 21 urges the shaft 15 rearwardly, and the shaft 15 isrestrained from a rearward motion by a stop ring 42 fitted in acomplementary groove of the shaf-t 15 at a point immediately to the`front of the forward end of the bushing 7. The reaction forces of thespring 31 are along a diameter of the shaft 15 and are applied in equalamounts at opposite sides of the shaft 15. The application of springpressure through a spring fulcrum 32 presents a torque urging a tilt ofthe shaft 15 Within its bushing 7. However, since each abutment 21 is alike radial distance from the shaft 15 and the force at each fulcrum 32is of like amount the resultant net torque presented is near or at thevanishing point, the torques cancelling one another.

Since the shaft 15 is not subject to any substantial net force urging atilt of the shaft within the bushing 7 a person may grasp the externalend of the control shaft 15 and adjust the position of the brush 20, andthen release his grip upon the shaft 15 without an attendant tiltingshift of the shaft due to internal forces created lby the bias spring31. The brush 20, consequently, will remain in the position to which itwas adjusted. This aspect of the invention is of particular advantage inapplications calling for precise adjustments of the brush 20, andwherein slight brush movements cause relatively substantial changes incircuit parameters. Further, bearing life between the control shaft 15and the bushing 7 will be materially enhanced by the elimination oftorques urging a tilt of the shaft. adjustment will then be had over agreater period of life, and the apparatus, consequently, will give agreater duration of service.

In the practice ofI the invention the elimination of torque, that wouldotherwise urge a tilt of the shaft, is accomplished by having thereactive forces of the bias spring 31 upon the brush carrier 16 be ofsuch magnitude and applied in such positions that the torques set upthereby cancel one another. In the form of the invention shown in thedrawings, the reaction forces of the spring 31 are applied at the twopositions of the diametrically opposed fulcrums 32. These positions, inaddition to being diametrically opposite one another, are equallydistant from the center of the shaft 15 and the forces are equal. As aconsequence, the net torque is either zero or close thereto, since theindividual torques 'are equal and opposite in direction. With theelimination Accuracy and precision of 6 of such internally developedtorque the shaft 15 will not tend to seek out any particular position,so that upon selecting a position for the control shaft 15, it will notstray from that position.

Having the brush 20 carried by the spring 31 further enhances thereliability of the apparatus. The relative shift, in a direction normalto the rear face of the base 1, that may occur between the brush 20 andthe spring fingers 34 will be minimized, as contrasted with a structurein which the spring is an element wholly separate from the convergingfaces between which the brush is urged. In the present invention thebrush 20 will, at all times, be pressed against the base 1 with fullspring pressure. Further, each of the brush feet 40, 41 will ridesmoothly along the conductive tracks 2, 3. A point to point contactbetween the crowned portion 38 of the brush 2t) and the ridge 39 of thespring 31 is retained, regardless of any tilt of the spring 31 that mayoccur, thereby insuring that the brush 20 rides along the face of thebase 1 with each of the three feet 40, 41 bearing with a substantialpressure against the base 1. This minimizes the introduction of noise inhigh gain amplification circuits.

To provide for electrical connection with the conductive tracks 2 and 3metallic leads 43, 44 and 45 are provided. One end of each of the leads43, 44, 45 is embedded within the base 1 at the time of molding, andeach embedded lead end extends to within a conductive track. Thus, theembedded end of the lead 43 is disposed within one terminus of thearcuate resistance track 2, the lead 44 has an end embedded in theopposite terminus of the resistance track 2, and the lead 4S has one endembedded in the collector track 3. Each lead 43, 44, 45 extendsdownwardly from the base 1 to present an array as shown in Figs. 2, 3and 5. Each lead 43, 44, 45 is quite thin in the fore and aft directionto permit ready bending for forming the embedded ends of the leads, andfor allowing the free ends to be readily moved forwardly toward thecasing wall 11. Each of the leads 43, 44, 45 is quite thick in thedirection paralleling the plane of the casing wall 11, so that the leadsmay not be readily bent toward one another to cause unwanteddisplacement from the desired spaced positions shown. Each lead 43, 44,45 is connected at its lower end to a terminal 46 of substantial sizeand dimension to present a strong, rugged means for attachment withexternal circuit leads. As shown in Figs. 2 to 5, the terminals 46 arepositioned in `a horizontal row at the bottom of the apparatus and byreason of the rigidity of the leads 43, 44, 45, in a direction paralleling the wall 11, the terminals 46 are held in spaced positions. Eachterminal 46 has a base plate 47 disposed against the back face of thehousing wall 11, to which the associated leads 43, 44, 45 is fixed. Eachof the base plates 47 is provided with a set ofears 48 protruding at theopposite sides. Each terminal 46 continues downwardly from its baseplate 47 and then turns rearwardly and upwardly at an oblique angle topresent a connection flange 49 that receives a terminal screw 50.

The lower portion of the cover 12 is formed with openings and recessescomplementary to the configurations of the terminals 46, as will now bedescribed. The case 12 has a horizontal enclosing wall 52 from which aset of four spaced vertical ribs 51 extend downwardly so that apartitioned space is provided for each of the terminals 46. Rcesses 53are formed in the forward edges of the ribs S1, each recess 53 having -aconfiguration mating that of a terminal ear 48. The depth of therecesses 53 corresponds to the thickness of the terminals 46, so thatupon disposing the cover 12 against the front wall 11, with theterminals 46 in place, the terminals 46 are rigidly held and areimmovable with respect to the casing formed by the cover 12 and fronthousing wall 11. Further, the spacing between the ribs 51 matches thewidth of the connection flanges 49 of the terminals 46 to restrictturning of the terminals upon application of twisting torqueto theterminal screws 50. Since movementof the terminals 46 is restricted topreclude shifting with respect to the cover l2 and housing wall il theterminals 46 are mechanically isolated from the internal electricallyconductive elements of the base plate l and the carrier 16. Inthis'fashion, high peak stresses and Shock blows may be delivered to theterminals 46 upon making connections with other circuit components,which Will not be transmitted to the precise electrical eiements housedwithin the cover l2.

The mechanical isolation of the terminals 26 is accomplished bypresenting terminal surfaces facing rearwardly vand forwardly, and alsoupwardly, downwardly and to both sides, all of which surfaces havemating counterparts in closely conforming surfaces of the cover 12 andhousing wall 11. By transmitting stress from the terminals 46 directlyto the members il and 32 the apparatus becomes ideally suited forindustrial applications. The use of precise variable resistanceapparatus can, therefore, be extended to elds not heretofore open forsuch apparatus.

We claim:

l. In a variable resistance apparatus having an arcuate resistance trackand a rotatable operating shaft concentric with the track, thecombination of a contact brush in engagement with the resistance trackthat is rotatable with the shaft and has a pair of radially extendingoutwardly converging side surfaces together with a back surface slopingradially outwardly away from the base; a brush biasing spring rotatablewith said shaft having a pair of radially extending outwardly convergingretaining sur-- faces complementary to the side surfaces of said brushbetween which said brush is disposed to be housed by and rotated withsaid spring; and a brush urging portion for said spring bearing upon thesloping back of said brush to thereby urge the brush against theresistance track and radially outward as said shaft, spring and brushare rotated to firmly retain the brush between said retaining surfaces.

2. In a variable resistance apparatus the combination comprising a baseincluding `a resistance track; a plurality of leads each electricallyconnected at one end to the resistance track and extending from thebase; a terminal for each of said leads electrically secured thereto,each terminal having a mounting portion with retaining surfaces facingforwardly and rearwardly, upwardly and downwardly, `and to each side,each terminal extending from its mounting portion with a screw receivingconnecting end that turns forwardly and across the front of the mountingportion to provide for a shank of a screw to be disposed between themounting portion and connecting end with the screw head exposed to thefront of the terminal; a housing member supporting said base; and acover movable into abutting relation with the housing member to form acasing for the base, and removable from such abutting relation with thehousing member; said cover and housing member when in abutting relationhaving a set of faces for each terminal which match each of theretaining surfaces of the associated terminal and closely conformtherewith to retain the terminals in fixed relation with the housingmember `and cover.

3. ln a variable resistance apparatus the combination comprising a basehaving conductive track areas; a plurality of leads each electricallyconnected to the conductive areas and extending from the base in side byside relation, each lead being relatively stiff in a sidewardlydirection and flexible in the direction normal to the sidewardlydirection; a housing member having a flat wall portion extendingalongside the leads; a terminal joined to each lead having a hat faceadapted to be placed against the flat wall portion of Said housingmember, and each terminal further having retaining surfaces facing toeach side, upwardly and downwardly and in the direction opposite thehousing member; and a cover all) ci adapted tobe brought up against thehousing member which has face areas matching and closely conforming withthe retaining faces of the terminals thereby holding the terminals fixedwith respect to the cover and housing member.

4. in a variable resistance apparatus the combination comprising a base;a circular resistance track carried by said base; a rotatable operatingshaft concentric with said resistance track; a conductive brush disposedradially from and rotatable with said shaft that is in bearingengagement with said resistance track, which brush has a pair ofradially extending outwardly converging side surfaces together with aback surface sloping radially outwardly away from the base; a brushbiasing spring having a portion urging said brush against the resistancetrack and extending circumferentially about the shaft to a positiondiametrically opposite said portion, said biasing spring having asliding engagement with said base at the position opposite said portion,and said portion of the spring urging the brush against the resistancetrack having a pair of radially extending outwardly converging retainingsurfaces that face one another and are complementary to the sidesurfaces of said brush between which said brush is disposed to becarried by and rotated with said spring, said spring portion also havinga brush engaging portion bearing upon the sloping back to said brush tothereby urge the brush against the resistance track with a forcecomponent in radially outward direction to retain the brush between saidretaining surfaces; and a pair of diametrically opposite spring engagingabutments extending radially of the shaft in quadrature with respect tosaid brush urging portion, each of which bears against said spring onthe side opposite said base to thereby stress the spring and urge boththe brush and the sliding engagement against the base, whereby thereaction forces of the spring against the abutments create torques thatindividually urge a tilting of the shaft and which cancel one another.

5. ln a variable resistance apparatus the combination comprising a base;a shaft rotatable with respect to said base; a resilient Spring memberin the form of a closed ring with an open center that encircles saidshaft and is spaced from the base; a contact brush inserted bctween thespring and the base which moves with the spring and bears against thebase in response to spring pressure, which brush has a pair of radiallyextending outwardly converging said surfaces together with a backsurface sloping radially outwardly away from the base; brush retainingmeans forming an integral part of said spring that has a pair ofradially extending outwardly converging retaining surfaces complementaryto the side surfaces of said brush between which said brush is disposedto be Ihoused by and rotated with said spring, which means furtherincludes a brush urging portion bearing upon the sloping back ot saidbrush to urge thc brush against the resistance track and radiallyoutward to lirrnly retain the brush between said retaining surfaces; abearing portion on said spring that bears against and slides across saidbase in response to spring pressure that is at a position diametricallyopposite said brush; and a pair of diametrically opposite abutments onsaid shaft in quadrature with said brush and bearing portion that 'bearagainst the spring at positions that arc at equal radial distances fromthe shaft axis.

6. in a variable resistance apparatus the combination comprising base; aShaft rotatable with respect to said base; a resilient spring member inthe form of a closed ring with an open center that encircles said shaftand is spaced from the base; a contact brush inserted between the springand the base which moves with the spring and bears against the base inresponse to spring pressure; a bearing portion on said spring that bearsagainst and slides across said base in response to spring pressure thatis at a position diametrically opposite said brush and at a radialdistance from said shaft substantially the same as the mean point ofapplication of the forces between the brush and base; and a pair ofdiametrically opposite abutments on said shaft in quadrature with saidbrush and bearing portion that bear against the spring at positions thatare at equal radial distances from the shaft axis.

7. In a variable resistance apparatus the combination comprising a base;a shaft rotatable with respect to said base; a resistance track on saidbase concentric with the shaft; a collector track on said baseconcentric with the shaft and spaced from the resistance track; aresilient spring member in the form of a closed ring with an open centerthat encircles said shaft and is spaced from the base; a contact brushinserted between the spring and the base that spans between theresistance and collector 15 2,135,809

tracks and which moves with the spring and bears against the base inresponse to spring pressure; a bearing portion on said spring that bearsagainst and slides across said base along a path between the resistanceand collector tracks in response to spring pressure and that is at aposition diametrically opposite said brush; and a pair of diametricallyopposite abutments on said shaft in quadrature with said brush andbearing portion that bear against the spring on the side thereofopposite said base and act upon the spring at positions that are atequal radial distances from the shaft axis.

References Cited in the le of this patent UNITED STATES PATENTS1,926,196 Da Costa Sept. 12, 1933 Fruth Nov. 8, 1938 2,576,044 Richmondet al Nov. 20, 1951 2,705,273 De Bell Mar. 29, 1955 UNITED STATES PATENTOFFICE Certificate of Correction Patent No. 2,868,931 January 13, 1959William F. Dehn et al.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction and that the saidLetters Patent should read as corrected below Column l, line 65,for.disenga.ging read -disengages-; column 8, line 26, for to said readof said-5 line 47, for said read -side-.

Signed and sealed this 28th day of April 1959.

[SEAL] Attest: T. B. MORROW, ROBERT C. WATSON, Attest/ng Ocer.Gommzssz'oner of Patents.

UNITED STATES PATENT OFFICE Certificate of Correction Patent No.2,868,931 January 1B, 1959 William F. Dehn et a1.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction and that the saidLetters Patent should read as corrected below Column l, line 65, fordisengaging read -dsengages-5 column 8, line 26, for to Said read ofsaid-; line 47, for said read -side-.

Signed and sealed this 28th day of April 1959.

[SEAL] Attest r T. B. MORROW, Attestng Oycer.

ROBERT C. WATSON, Uommzssz'oner of Patents.

